The present invention relates generally to an improved steering system for a tractor or other vehicle of the type having a pair of driven wheels and a third, freely rotatable castor or tail wheel or wheels. More particularly, the present invention relates to a steering assist device for use in connection with such a vehicle for mechanically turning the tail or castor wheel to compensate for side hill drift and to assist in turning the vehicle when it is driven on level ground.
Tractors or other power vehicles of the type with which the present invention has specific application have been in existence for many years. Such tractors are commonly used as orchard tractors and generally include a pair of forwardly disposed driven wheels and a rear, freely rotatable tail or castor wheel. These tractors are not steered in the conventional manner by mechanically rotating one or more of its wheels about a vertical axis. Rather, steering of the vehicle is controlled by the application of differential hydraulic flow to the respective drive wheels. For example, by providing a greater hydraulic flow to the drive motor for the right drive wheel, the right wheel will tend to move faster than the left, thus resulting in the tractor turning to the left. The rear castor wheel, in these vehicles, of course, merely trails the pair of drive wheels and serves primarily to support the rearward end of the vehicle.
One of the problems that has existed with respect to tractors and power vehicles of this sort is that when they are driven on the side of a hill, the rearward end, the end supported by the castor wheel, tends to roll down the hill. Thus, the drive wheels at the forward end have two jobs. One is to help drive the tractor forward, the second is to hold the rearward end of the tractor on the hill. This is particularly true of the drive wheel on the uphill side which must pull and hold the rear end of the tractor up on the hill while the lower wheel acts a pivot point. Thus, greater hydraulic pressures are needed to drive the uphill wheel, while the lower wheel need only develop enough pressure to help drive the machine forward. Although the hydraulic pump and motor for the uphill wheel is generally capable of producing enough pressure and driving force to perform both jobs, the problem has been that it does not have enough traction for both jobs. Because of the force tending to move the rearward end of the tractor down the hill and the distribution of weight being concentrated on the downhill wheel, the upper wheel tends to spin. Thus, the lower wheel goes forward, the upper wheel cannot go forward because of poor traction and as a result the rear end of the tractor swings down the hill.
The above-mentioned problem, while more pronounced when the tractor is being driven on the side of a hill, also exists when the tractor is being turned on level ground. When the tractor is moving straight ahead, the only job of the drive wheels is to move the vehicle in a forward direction. When the tractor is turned, however, one of the drive wheels is driven faster than the other, thus causing the tractor to pivot relative to the other drive wheel and the rearward end of the tractor to swing around. Thus, in turning the tractor on level ground, the outside drive wheel must not only continue to move the vehicle forward, but must exert additional driving force to swing the rearward end of the vehicle in the manner mentioned above. Because of this, the outer drive wheel often slips when turning and the steering is generally not as responsive as desired.
Accordingly, there is a real need for a steering system and control and in particular a steering assist device which will alleviate the side hill drifting tendency of a tractor of the type described above and which will also eliminate similar problems when turning such vehicle on level ground.